Introduction

LPA is a lysophospholipid that regulates multiple biological processes, including proliferation, cellular morphology, and endothelial permeability. LPA is produced through hydrolysis of phosphatidic acids or cleavage of lysophospholipids, the different forms of lysophosphatidic acid bind with six well-studied LPA receptors (LPARs), transmembrane proteins whose activation leads to different cellular responses [1, 2]. These variable responses are dependent on which downstream proteins are recruited as well as their interactions. LPA levels are elevated in cases of pulmonary fibrosis, hinting at its role in promoting inflammatory effects within multiple cell types [3]. Outside of its inflammatory effects, the different signaling pathways induced by LPA play roles in development, cellular proliferation, differentiation, as well as apoptosis.

The Rho family of proteins are a branch of the Ras superfamily of small GTPases. They are critical participants in signaling cascades and pathways, hydrolyzing GTP to activate or regulate other proteins such as kinases [4, 5]. Rho GTPases are regulated by the enzymatic activities of GTPase-activating proteins, guanine nucleotide exchange factors (GEFs) and GDP dissociation inhibitors [6,7,8]. These GTPases help regulate cytoskeletal organization, cellular migration, proliferation, and apoptosis [4, 5]. As a signaling molecule, LPA can regulate these GTPases to elicit specific cellular responses (Fig. 1). In this review, we briefly examine the interactions and effects between LPA and RhoA, Rac1, Cdc42 from the Rho family of GTPases, characterizing their behaviors mainly within the context of endothelial and epithelial cells in lung tissue (Fig. 2).

Fig. 1
figure 1

Rho GTPases activiation by lysophosphatidic acid (LPA). Upon LPA binding, LPARs undergo conformational changes and induce activation of heterotrimeric G proteins. GEFs serving as the direct downstream effectors of heterotrimeric G proteins acivates Rho GTPase by simulating formation of the GTP-bound state. LPARs, receptors for LPA; G protein heterotrimeric complex: α or βγ subunits. Scheme created using Biorender.com

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Fig. 2
figure 2

LPA can induce a variety of processes or functions through interactions with one of the 3 GTPases from the Rho Family, as outlined in this graphic. These proteins’ functions are not mutually exclusive or isolated from one another, some working together to achieve overall cell motility or migration. Scheme created using Biorender.com

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LPA Regulates RhoA Activation

RhoA, along with Rac1 and Cdc42, are regulators for the actin cytoskeleton within endothelial and epithelial cells, affecting a variety of cellular functions [9]. RhoA is known to regulate formation of contractile stress fibers in non-muscle cells, which enable cellular migration and movement [10, 11]. RhoA also is involved in regulation of angiogenesis and extension wound repair driven by LPA, being essential for such within in vitro endothelial cells but partly optional for in vivo cells [12]. When activated by LPA/LPARs, RhoA and Rho kinase may contribute to dysfunction of the endothelial barrier [13, 14]. Activation of RhoA mediates the phosphorylation of myosin light chains within actin-myosin bundles found within non-muscle cells, leading to contractile movements at cell peripheries. These contractions permit the formation of gaps between cells, such as the endothelial cells lining blood vessels [15]. The permeability of the endothelial barrier is temporarily increased, but this induced hyperpermeability is less effective than the degree of hyperpermeability induced by thrombin [13]. Inducible hyperpermeability plays a role in inflammation, permitting infiltration by other cell types through the barrier.

RhoA is also involved in the assembly and development of the apical cell surface in multiciliated epithelial cells, such as those lining the proximal and larger airways [16]. Further down the airway, the alveolar epithelium consists of type II and type I cells, where type II produces surfactant and type I composes gas exchange surfaces [17]. Breathing is a cyclic motion that can trigger stretch-induced cell signaling, which can result in a variety of effects such as increased ion channel expression or activation of substances like transforming growth factor-β (TGF-β) [18, 19]. As a result of this signaling, type II cells are able to transition into type I cells, a process mediated by the Rho protein family [20]. A previous study using Normal Human Bronchial Epithelial cells (NHBE) found that LPA induces TGF-β activation [3]. Upon LPA binding to receptor LPA2, a signaling cascade is initiated through RhoA and Rho kinase, affecting the ανβ6 integrin and subsequently activating TGF-β. This activation was found to be dependent on LPA concentration. As TGF-β is an important mediator of lung injury and fibrosis [21,22,23], LPA and RhoA can play profibrotic roles.

LPA Regulates Rac1 Activation

Rac1 shares similar functions to RhoA, such as reorganizing the actin cytoskeleton, cell migration and transformation [9]. Activation of Rac1 triggers the formation of a lamellipodia, a projection of actin which provides a means of cell motility [24]. Studies with Rac1 knockout mice demonstrated reduced and defective migration of neutrophils across membranes, such as the endothelial barrier [25]. In comparison to wild-type mice, loss of Rac1 in neutrophils exhibited a reduced number of neutrophils infiltrating lung interstitial and alveolar spaces. In addition, there was abnormal localization of other substances such as RhoA or improper assembly or function of actin filament structures [25]. It is also interesting to note that LPA can increased oxidative stress through increased reactive oxygen species production in pulmonary microvascular endothelial cells [26]. This study demonstrated that this effect was achieved mainly though increased activation of NOX2, which coincided with Rac1 activation and translocation to the plasma membrane. Inactivation of the LPA receptor via inhibition or knockdown diminished NOX2 activity, demonstrating another interaction between LPA and Rac1 [26]. In another study utilizing mouse lung epithelial cells, decreased expression of Rac1 through overexpression of FBXL19 lead to reduced epithelial cell migration following stimulation by LPA [27].

LPA Regulates Cdc42 Activation

Activation of Cdc42 triggers filopodia extensions, cytoplasmic projections along the leading edge of the lamellipodia structure formed from Rac activation [28]. A previous study demonstrated how upon stimulation via LPA treatment, Cdc42, alongside Rac, were essential to endothelial cell spreading [29]. Knockout of either GTPase resulted in rounded cells which did not spread effectively following LPA stimulus, a result which was similarly achieved by inhibiting LPA itself through use of pertussis toxin. Another study found that outside of changes in actin distribution or structures, Cdc42 also mediated the reorientation of microtubule-organization centers (MTOC) in fibroblasts and endothelial cells following stimulation by LPA [30]. It was seen that Cdc42, but not Rho or Rac, was necessary and sufficient for MTOC reorientation, a necessity to permit the formation of stable microtubules along the leading edge of a migrating cell. The effects of LPA on Cdc42 are important to cellular migration.

Conclusion

Lysophosphatidic acid is an important bioactive molecule which contributes to many important cellular processes. Through its interactions with RhoA, Rac1, and Cdc42, we see that LPA induces mechanisms and pathways which lead to the assembly of structures necessary to cellular migration and mobility. Some effects of LPA contribute to or permit the infiltration by some cells into other tissues, as seen with endothelial barrier permeability and neutrophils. The effects and processes in which LPA partake in as a signaling molecule are important components to inflammatory responses, demonstrating one of the many important roles that lysophosphatidic acid plays.